Image printheads used in xerographic recording systems are well known in the art. The printhead generally includes a linear array of a plurality of discrete light emitting sources optically coupled to a linear lens array. Light emitting diode (LED) arrays are preferred for many recording applications. In order to achieve high resolution, a large number of light emitting diodes (LEDs) are arranged in a linear array and means are included for providing a relative movement between the linear array and the photoreceptor so as to produce a scanning movement of the linear array over the surface of the photoreceptor. Thus, the photoreceptor may be exposed to provide a desired image one line at a time as the LED array and associated lens array is advanced relative to the photoreceptor either continuously or in stepping motion. Each LED pixel in the linear array is used to expose a corresponding area on the photoreceptor to a value determined by image defining video data information. Where the LEDs are arranged in a row at say 600 LEDs to the inch approximately 5000 LEDs may be present upon a printhead.
In a color xerographic system, as described U.S. Pat. No. 5,192,958, a plurality of LED printheads may be positioned adjacent the photoreceptor surface or photoconductor and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth print bar may be added if black images arc to be created as well.
The arrays are addressed by video image signals whose application is controlled by a control circuit. Each array is optically coupled to focus the emitter outputs to form three spaced latent images on the surface of a photoreceptor belt. The optical coupling is accomplished by a plurality of gradient index lens arrays; the lens array sold under the name SELFOC.TM. a trademark of Nippon Sheet Glass Co., Ltd. Upstream of each exposure station, a charge device places a predetermined charge on the surface of the belt. Downstream from each exposure station, a development system develops a latent image of the last exposure without disturbing previously developed images.
With such a system as that disclosed, each colored image must be precisely aligned such that all corresponding pixels in the image areas are registered. The printhead alignment requirements are that the LEDs of each printhead must be aligned in the main scan or X-direction so that each active write length is equal. The printhead must also be aligned in the skew or Y-direction and in the Z-direction (tilt). This alignment must be maintained through continuous revolutions (passes) of the photoreceptor.
To maintain exact color registration of each image, the overall length of the write area, the pixel to pixel placement, and the straightness of the image line must all be within the required exacting tolerance.
A specific problem in correcting exact image-to-image registration, which is addressed by the prior art, is the change in length that an LED array undergoes when subjected to temperature increases, which are caused either by heat generated internally to the array, or by heat absorbed by the array from surrounding machine environment.
Typically, accurate LED arrays are made on a single ceramic substrate. To achieve proper registration, according to the prior art, the temperature of all LED arrays used in the printhead is allowed to vary over a relatively large temperature range. The technique described in the prior art is to keep them all at the "same" temperature. This way, the overall write length of the arrays will increase or decrease at the same time and at the same rate, thus achieving individual registration at every pixel.
The prior art accomplishes this objective by employing a manifold subframe that is adapted to securely mount plural printheads in parallel and perpendicular alignment. The subframe has apertures therethrough for circulating a cooling medium through the subframe and through the interior of arrays, the circulating medium maintains the arrays and the subframe at the same temperature.
A problem with the approach suggested in the prior art is that in coupling all the printheads rigidly to the manifold subframe, no ability is provided to adjust the LED print bars in the Y and Z directions which are perpendicular to the X direction. Thus registration of color separation images can be a problem with the approach suggested by the prior art.